1. Field of the Invention
This invention relates to an improvement in integrated circuit bonding pads, and more specifically to a circuit for eliminating stray currents in those integrated circuit bonding pads which are not bonded to a pin in the integrated circuit package or otherwise are not externally connected.
2. Description of the Prior Art
It is often desirable with regard to integrated circuit chips to provide chips (i.e. die) which can be housed in different integrated circuit packages having varying numbers of pins (external terminals). As is well known, integrated circuit packages are available in a variety of standard number of pins. It is often desirable to package a particular type of integrated circuit (IC) die in several packages having different number of pins; for instance a given die may be designed to be housed in both an 84 pin and a 68 pin package. In the latter case, 16 of the die pads on the integrated circuit are left unbonded, i.e. no external connection is made to those pads. If such an unbonded pad is an input pad, which ordinarily would receive signals from the outside, this poses a problem because the voltage at such an inaccessible pad may take on intermediate values (i.e. the input is floating) which in turn causes excessive current to be drawn by the circuitry on the die connected to that pad.
Another problem associated with such an unbonded pad is functional failure of the IC due to intermediate voltage values occurring within the chip which are caused by the unbonded pad. That is to say, such an unbonded pad may provide what appears to be input signals to the chip, which prevent the IC from operating properly. The pad voltage floats by taking on intermediate values due to capacitive coupling from adjacent connected traces on the IC, or slow leakage paths from nearby integrated circuit nodes.
It is a known technique to use a passive electronic device such as a resistor to "pull" the pad voltage of such an unconnected pad either always high or always low if no external signal is provided, i.e. no external drive. The disadvantage of this prior art technique is that direct current is drawn by the passive device even when the pad is driven to high or low voltage.
In prior art FIG. 1 input pad 10 of an integrated circuit is connected to a series of input buffers 12 and 14 which are ultimately connected to the IC logic circuitry. The conventional input buffers (inverters) 12 and 14 are (as is usual) each a pair of transistors, typically field effect transistors one P-channel and one N-channel with their gate terminals connected together and the drain of one transistor connected to the source of the other, providing therefrom the output signal. The other source and drain terminals of the two transistors are connected respectively to a source of voltage and to ground. Connected between pad 10 and the first input buffer 12 is a first terminal of pull down resistor 18 having its second terminal connected as shown to ground. Alternatively the second terminal of resistor 18 is connected to a voltage source, and therefore resistor 18 is a pull up resistor. As can be seen, if pad 10 is driven to a high voltage from an external signal source connected thereto, resistor 18 draws current.
No satisfactory solution therefore is known to this unbonded pad problem in the prior art, without drawing direct current. Drawing direct current is undesirable, especially for instance in battery powered equipment.